Conventionally, a steel for machine structural use, such as S48C etc. regulated in JIS has been employed for a crankshaft and the like used for an automobile, an industrial vehicle and the like, since they require wear resistance and fatigue strength. The said S48C is a so-called “heat treated steel”, therefore, the wear resistance and the fatigue strength thereof are improved by performing the following treatments. That is to say, first, in order to attain a predetermined strength, a quench hardening and tempering treatment after hot working is performed. Second, the said quench hardened and tempered S48C is made into a predetermined shape by machining or the like. Finally, an induction hardening treatment to necessary parts is carried out in order to form a surface hardened layer.
However, the heat treated steel, as described above, consumes a lot of energy and labor hours, and also requires high facility cost because the quench hardening and tempering heat-treatment is performed after hot forging. In recent years, therefore, the development of a non-heat treated steel usable in a hot forged state has been actively carried out in order to respond to the social need of energy saving, and several reports for non-heat treated steel for induction hardening have been made.
For example, the Patent Document 1 discloses “a non-heat treated steel for induction hardening, which comprises by weight percent, C: 0.30 to 0.60%, Si: 0.03 to 1.0%, Mn: 0.5 to 2.0%, and further one or two of Mo: 0.05 to 0.5% and Nb: 0.01 to 0.3%, with the balance being substantially Fe, and having a microstructure in which the volume ratio of bainite is regulated to 75% or more”, and the like.
Further, the Patent Document 2 discloses “a non-heat treated steel for induction hardening, which comprises by weight ratio, C: 0.30 to 0.60%, Si: 0.10 to 0.80%, Mn: 0.60 to 2.00%, Cr: 0.60% or less, V: 0.05 to 0.30%, Al: 0.030 to 0.100%, N: 0.0080 to 0.0200% and B: 0.0005 to 0.0050%, with the balance being Fe and impurities”, and the like.
Patent Document 1: Japanese Laid-Open Patent Publication No. 63-100157
Patent Document 2: Japanese Laid-Open Patent Publication No. 2-179841